Method for controlling flux pressure during a sintering process



g- 13, 1964 G. CHEROFF ETAL 3,145,120

mz'mon FOR CONTROLLING FLUX PRESSURE DURING A SINTERING PROCESS Filed Feb. 12, 1962 2 Sheets-Sheet l PHASE DIAGRAM FOR THE SYSTEM Cd Se CdCl #1255 FIG. 1 1200 H00 LIQUID +VAPOR 00 Se (Crystals) LIQUID+ VAPOR TEMPERATURE 600 b 535 .4561" E64 l a 500 I 0/ C) 522 Cd (:1 (crystals) 400 +L|QUID VAPOR l l 500 200 Cd Se(crys1o|s)+ Cd C12 (crystals) i VAPOR 10o g i 1 l 10 so so so 10 so so INVENTORS GEORGE CHEROFF FREDERICK HOCHBERG ARNOLD REISMAN SOL TRIEBWASSER 8 F I G. 3 BY ATTORNEY 18, 1964 G. CHEROFF ETAL 3,145,120

METHOD FOR CONTROLLING FLUX PRESSURE DURING A SINTERING PROCESS Filed Feb. 12, 1962 2 Sheets-Sheet 2 nitc ate This invention relates to a method for controlling the partial pressure of a volatile fiux, volatile at the temperature during which a sintering process is to be effected. More particularly, it describes a process for fabricating photoconductor sintered layers which incorporate flux partial pressure control during the sintering cycle.

It has been found in the past that in any sintering process during which a flux is used that if the flux mixed with the sintering material is lost only a partial sintering occurs which results in sintered material having non-reproducible electrical characteristics.

Ordinarily, fluxes are employed to decrease the temperatures necessary for the formation of a liquid phase which enhances and facilitates sintering, grain growth and inter particle connection. In general, fluxes may be used in the sintering of any material. Normally fluxes are used in the sintering of such materials as for example, cadmium selenide or cadmium sulfide, which are semiconductors as Well as photoconductors. The fluxes used in the sintering of photoconductor materials are generally of high volatility in the temperature regions at which sintering must be effected. Consequently, relatively large quantities of flux are added to the initial unsintered bulk materials. However, the amounts that can be initially added are distinctly limited or else severe dilution of the photoconductors results and thus the sintered layers are thin and noncontinuous. Although the problem exists regardless of the size of the individual photoconductor element, the problem becomes more acute with decreasing size since the absolute quantity of flux in the original mixture is soon exhausted before effective sintering can occur. Thus the net effect is to yield photoconductor elements with marked non-reproducibility which when part of multiple element devices greatly reduces the device yield since in general, all elements on multiple element devices must fall within the desired specifications. In the past it has been diflicult to maintain appropriate flux material vapor pressures of the sintering photoconductor during the latters heat treatment with the result that the electrical properties of the elements of a multiple element device vary markedly within the device itself and from device to device rendering reproducibility difficult.

The process of the invention controls the flux vapor pressure in the ambient atmosphere of the sintering photoconductor material which in turn markedly affects the electrical and physical properties of the resulting sintering photoconductor. This process automatically maintains the desired flux material vapor pressure over the sintering photoconductor during the latters heat treatment at all temperatures involved in the temperature sintering cycle, and results in multiple element photoconductor devices whose properties do not vary within the device itself or from device to device. Similar results are obtained with single element photoconductor devices processed in the same manner.

The process of the invention also consists of sintering cadmium selenide in a controlled gaseous atmosphere (oxygen and nitrogen) at a temperature dictated by the phase diagram for the system cadmium selenide-cadmium chloride for a composition Where approximately l20 mol percent of the cadmium selenide is in a liquid phase.

The incorporation of a precise method of vapor pressure control of the normally volatile flux material during the sintering cycle also maintains the flux concentration close to its initial concentration during the entire sintering cycle.

It is an object of the invention to control the partial pressure of the flux during any sintering process in which a volatile flux is used.

It is another object of the invention to control the flux partial pressure during the fabrication of fluxed sintered layer photoconductors.

it is an object of the invention to control the cadmium halide flux partial pressure during the fabrication of fiuxed sintered layer photoconductors.

A further object of the invention is to control the cadmium chloride fiux partial pressure used in the fabrication of fluxed cadmium selenide sintered layer photoconductors.

Still another object of the invention is to control the cadmium chloride flux partial pressure used in the fabrica tion of fiuxed cadmium sulfide sintered layer photoconductors.

It is an object of the invention to control the cadmium chloride flux partial pressure used in the fabrication of fluxed solid solutions of cadmium selenide and cadmium sulfide sintered layer photoconductors.

Another object of the invention is to control the cadmium bromide or cadmium iodide flux partial pressure during the fabrication of fluxed sintered layer photoconductors of cadmium selenide or cadmium sulfide or solid solutions of cadmium sulfide and cadmium selenide.

it is another object of the invention to control the temperature range of sintering so as to provide controlled amounts of the liquid phase fluxed photoconductor material during the sintering cycle of the photoconductor sintered layer fabrication process.

A further object of the invention is to control the sintering temperature range of cadmium selenide fluxcd by cadmium chloride so as to provide controlled amounts of the liquid phase fluxed photoconductor material during the sintering cycle of the photoconductor sintered layer fabrication process.

Still a further object of the invention is to control the sintering time range for a given temperature cycling of sintering so as to provide uniform controlled grain growth size Within the sintered layer photoconductor.

Also an object of the invention is to control the ambient atmosphere other than flux atmosphere so as to enable continuous (self adjusting) vacancy compensation of photoconductor sintered layer at all temperatures encountered during the sintering cycle of the fabricated process.

The foregoing and other objects, features, and advantages of the invention will be apparent from the following more particular description of a proven embodiment of the invention, as illustrated in the accompanying drawings.

FIGURE 1 is the CdCl2.

FIGURE 2 is a top sectional view of apparatus used in the process of the invention.

FIGURE 3 is an enlarged perspective view of one of the sample carriers of the apparatus.

All chemically stable compounds are potential fluxes. A flux is hereby defined as a material which in admixture with one or more other materials lowers the temperature at which a liquid phase is formed for any of the individual admixed substances present in the pure state. Aside from the possible action as a controlled source of impurity atoms, the primary functions of a flux are to depress the melting point of the material with which it is admixed.

For example CdSe melts at 1235 C.i4 admixing CdSe with CdCl a liquid phase will phase diagram for the system CdSe C. and by be formed at 522 C., some 700 below the temperature at which a liquid phase will form in pure CdSe. However, all known stable compounds exhibit an increase in vapor pressure with temperature. CdCl for example at 25 C. exhibits a vapor pressure of approximately mm. of Hg while at 522 C. it exhibits a vapor pressure of approximately 40 mm. of Hg. Since the formation of a liquid phase in the system CdSe-CdC1 occurs at 522 C. where the vapor pressure of CdCl is high, it is evident that the latter will volatilize to an appreciable extent in an open system at a temperature of 522 C. Other fluxes such as for example CdBr and Cdl will behave in a similar manner.

The eifects of volatilization of a flux during a sin-tering process can best be explained by considering the phase diagram of the system CdSeCdCl shown in FlGURE 1. This diagram shows the relationship between melting point and composition of the system. The ordinates, of which there are two, are either pure CdSe or pure CdCl The first of these (i.e. pure CdSe), melts at 1234 C. and its melting point is depressed to as low as 522 C. by the addition of the flux CdCl In fact, at approximately 80 mole percent CdCl such a mixture would completely melt at 522 C. At lower CdCl concentrations proportionally smaller percent-ages of the total mixture would melt at 522 C. while the remainder would melt at a higher temperature. The temperature at which complete melting occurs for any specified initial concentration is determined by a line coincident with the composition and parallel to the ordinate and the intersection with the curved line extending from 1234 C. to 522 C. (this curved line being the solubility curve of CdSe in 0.101

At-each composition and temperature line intersection, point b for example, a line parallel to the abscissa (line abc) intersects the CdSe pure composition line (at a point a) and the boundary between the liquid, vapor and solid CdSe, liquid-vapor regions (at point c) can be used to determine the fraction of the CdSe which is in the dissolved state. The left arm of such a line (the arm ab) divided by the length of both arms (ab-H30) is the fraction of CdSe present in the liquid phase.

If it is desired to effect approximately a solution of the selenide at 535 C., an initial mixture containing approximately 10 mole percent CdCl is added. When such a mixture is heated to 535 C., the CdCl will be volatilizing at a rapid rate and the composition'of the system will shift toward pure CdSe. The net result, of course, is that the amount of liquid phase will decrease and as a consequence of this, the amount of sintering that will occur in a given time interval at 535 C. will decrease. After a certain elapse of time, in fact, all of the CdCl will have volatilized and to all intents and purposes sintering will terminate. In order to take into account this loss of CdCl the initial quantity of CdCl may be increased, but this is practical only for a short range of compositions since the desired end product is sintered CdSe' in a continuous layer. If too large a quantity of flux is employed the desired material will be diluted to an extent such that the resulting layer is thin and discontinuous. For practical considerations a range of from approximately 5-20 mole percent of 'flux is generally useful. It is evident that as sintering is effected at higher and higher temperatures, the problem of flux volatilization increases with temperature. It is evident from the above therefore, that the preparation of a controlled uniform sintered'layer in a volatile flux system is difficult to achieve it nomeasures are employed to control flux volatilization during the sintering process. For practical considerations, it is desirable to elfect sintering at as low a temperature as possible so as to minimize contamination of the sintering material. The lowest practical temperatures are in the vicinity. of the eutectic melting temperature, for example, 522 C. for CdSe fluxed with CdCl For'a given system containing a specified quantity of flux and a specified quantity, of material to be sintered, when precautions are taken to retain approximately the initial concentrations of flux during the sintering cycle, the amount of grain growth that occurs is dependent on the temperature of the sintering cycle and the length of time sintering occurs in the presence of a liquid phase. Thus, to form sintered layers of CdSe using CdCl as a flux such that ultimate grain size is approximately 8 microns, a 1 micron powder is heated for approximately 20 minutes at 535 C. in a system containing 10 mole percent of flux. For this same starting composition and a sintering temperature of 575 C. only ltl minutes are required.

The above considerations are primarily concerned with effecting desired degrees of sintering. In the preparation of a photoconductor sintered layer as for example the fabrication of a CdSe sintered layer, the final electrical properties must be considered. These properties are to a considerable degree dependent on the vacancy concentration of each of the elements, Cd and Se in the lattice and more particularly with the excess of Se vacancies over Cd vacancies since CdSe is an n type semiconductor. At each temperature in the sintering cycle the number of such excess vacancies varies, the number decreasing with decreasing temperature. A final fabricated CdSe photoconductor element will when prepared at elevated temperatures have a much larger excess of Se vacancies than is thermodynamically required at room temperature and consequently if in the fabrication process no precautions are taken to back fill these vacancies the resulting sintered layer will he metastable at room temperature relative to its vacancy content. Such metastability manifests itself by making the fabricated device unstable since it will tend to absorb oxygen to fill in the excess Se vacancies. Such a device will also exhibit poor dark conductivity because the large number of excess metastable Se vacancies are uncompensated. Oxygen may be utilized to back fill the excess Se vacancies during the sintering process by conducting the sintering cycle in an oxygen-nitrogen atmos phere such that back filling occurs automatically during 7 the sintering cycle resulting in a stable compensated device. The range of oxygen-nitrogen composition must be restricted in order to prevent formation of appreciable quantities of a second phase of Cd on the surface of the sintered layer. The formation of such a second phase will result in photoconductors exhibiting unacceptable electrical scatter, especially in multielemen-t devices and will also result in photoconductors exhibiting nonlinear voltage-current characteristics present in a non-reproducible fashion and thus detract from the desired electrical properties of the photoconductor device.

The above discussion is equally applicable to sintering of CdS or solid solutions of CdS and CdSe using CdCl as a flux or to sintering of CdSe, CdS or solid solutions of both with fluxes generally and more specifically with other useful fluxes such as Cdhr or CdI The process of the invention which involves control of flux partial vapor pressure can best be demonstrated in the context of a total sintering process for example, that employed in fabricating sintered photoconductor layers. An embodiment of the invention will be described using CdSe fluxed with CdCl in a sintering cycle which represents one stage in a multistage fabrication process for sintered CdSe photoconductor layers.

A mixture of CdSe doped with Cu for example, and CdCl with the latter constituting 5-20 mole percent of the total mixture is affixed to a suitable inert substrate such as for example glass, alumina, quartz, ceramic material, etc. The substrate and unsintered materials are heated in a 0.2-1.7% by volume of oxygen in an oxygennitrogen gas mixture to a temperature of 522 C.600 C. for a period of 5-20 minutes at the peak temperature. The substrate with the unsintered materials are placed upon a volume restricting container such that the material to be sintered faces into the container. The volume of the container is chosen commensurate with the quantity of flux present in the unsintered materials such that only a small fraction of this quantity is required to fulfill the flux partial vapor pressure requirement at the sintering temperature. Alternately, the flux, CdCl is fused in a thin layer at the bottom of the container to 8 boats. The outer of these 8 is simply the carrier which is attached to the continuous chain by Wire books 9. The inner boat 15) is actually the sample holder. The substrate 11 is inverted over the inner boat and it is seen from FIG- provide an independent source of flux partial pressure. 5 URE 3 that a semi-tight seal is formed between the inner Alternately a bed of chromatographic grade A1 0 or boat (the volume restricting or flux pressure adjusting other inert finely divided material such as, for example, chamber) and the substrate. These sample carriers fill SiO etc. is superimposed upon the fused thin flux layer the entire length of the continuous chain so as to set up at the bottom of the container so as to enable control of steady state heat conditions in the furnace 12 in which the rate of flux evaporation from the source bed. Alterthere are heating coils 12a of FIGURE 2. The entire connately a mixture similar in composition to that being trolled atmosphere apparatus is valved 13 to permit equalsintered is fused at the bottom of the container and ization of gas flow into and out of the apparatus. The covered with a powder of A1 0 or other inert material. corner boxes 14, are water cooled by condenser coils 15 Alternately an unfused layer of either CdCl or mixture as are the connections 16 between these corner boxes and similar in composition to that being sintered is placed in 15 the furnace tube 17. The continuous chain is traversed the bottom of the container and either left uncovered or via a variable speed pulley 18 to enable traverse rates of covered by a bed of powdered A1 0 or other suitable from .14"/minute. Operating access to the glove box inert material. is via gloves 19 although loading and unloading of the The control of the flux partial vapor pressure at the samples is eifected Withavacuum pickup 20. Atmosphere sintering temperature in the fabrication process for pho- 20 circulation within the apparatus is elfected with a circulattoconductors is achieved in such a manner that it is selfmg pump 21. adjusting or automatic. The various methods by which EXAMPLE 1 the flux partial vapor pressure is self-controlled or selfadjusting are as follows: (Cdse fluxed Wlth Cdcl2) The fluX cadmium chloride 18 fqsed m thm layer .035 g. of CbSe are admixed with .0016 g. of 0.101 and at the bottom of the container to provide an independent 35X -3 mg. of Cu added as cuclz and affixed to a glass sauce of flux Pattlal Vapor ptesusuresubstrate in 7 rectangular shapes of equal surface area A bed of ehfematographte grade altlmlhum (Hilde in a conventional manner. This slide is placed face down 2 3) other lhetthhely dlvlded material 15 Supenm' on a container having a volume of 1% cubic inches such P Oh the fused thlh flux layer at the bottom of the that the entire area of the substrate upon which material eehtftllhel' so as to enable Control of the h of flux to be sintered is located faces into the interior of the con- Otatloh front the h' e (m1eeu1a r S1eVe)- tainer. The container and substrate are drawn into a A m1Xt11fe Similar 1h Composltlon to h bemg furnace having a peak temperature of 522 C. over a sintered fused the bottom of the e and length three times the substrate length in which a .2% 0 Y h a Whlte POWder of 2 3 other Inert 99.8% N atmosphere is maintained. The sintering is terial (restrict volume to prevent loss to ambient atmoseffected for 20 minutes The sintered layers are cooled phere). washed with water to remove remaining flux and elech h e layer Pt thm cadmtutn chtonde q troded. Grain size of the sintered layers are approximixture similar 1n composltlon to that being s ntered 1s 0 mately 6 microns in Size The particle Size of the Placed in the bottom of the eontamer and elther 4 sintered CdSe prior to the processing is less than 1 micron. eoYeted P Covered a b of Powdered A1203 or other I The electrical properties of layers processed in the Suitable lhett e same way each time are reproducible. These electroded The appatatus Much e Process of the mventfon sintered layer photoconductors are useful as active elemay be carned out 1s shown in FIGURE 2. It consists ments in computer logic circuits of an airtight glove box 1 to which are affixed gas locks 49 2 to permit access to the glove box via lock doors 3 with- EXAMPLES 248 out affecting the atmosphere. Passing through the glove (cdse fluxed with cdclz) box via a quartz tube 4 which is opened in its center 5 is an endless chain 6 to which are attached sample care Procedure 0f Example 1 1S repeated eXCePt the riers 7. The latter are shown in detail in FIGURE 3. 50 conditions or parameters of the process are varied as in- The sample carriers are seen to consist of two nested dicated in Table Iwith the corresponding result set forth.

Table I Weight Sinter- Photoof photoing time Starting Final Example conducconduc- Weight Temperat peak Composition oiarnbient atparticle sintered N0. tor ma- Flux tor rnaof flux ature tempermosphere size (miparticle terial terial (grams) 0.) ature crons) size (m (grams) (mincrons) utes) 01101 0. 035 0. 0016 522 20 0.5% or, 1 6 05012 0. 035 0. 0016 522 20 1.0% 0 1 0 CdClg 0. 035 0. 0016 522 20 1.7% 02, 1 0 0501 0. 035 0. 0016 535 15 0.2% 0 1 5 0501 0. 035 0.0010 535 15 0.5% 02, 7 1 6 0501 0.035 0. 0016 535 15 1.0% 02, 00.0% N2. 1 0 05012 0. 035 0. 0016 535 15 1.7% 02, 03.3% N 1 5 05012 0. 035 0. 0016 575 10 0.2% o2,90.s% 1 6 05101 0. 035 0. 0013 575 10 0.5% 02, 1 0 05101 0. 035 0. 0016 575 10 1.0% 02, 1 6 01101 0. 035 0. 0016 575 10 1.7% or, 1 5 05012 0. 035 0. 0016 500 5 0.2% 0 1 6 0501 0. 035 0. 0016 600 5 0.5% 0 1 5 00012 0. 035 0. 0016 000 5 1.0% 02, 1 6 0e01, 0.035 0.0016 500 5 1.7% 02, 1 5 (3001;, 0. 035 0. 0037 522 20 0.2% 02, 1' 3 0501 0. 035 0. 0037 522 20 0.5% 02, 1 8 01101 0. 035 0.0037 522 20 1.0% 0 1 3 0001 0. 035 0. 0037 522 20 1.7% 0 1 8 00012 0. 035 0. 0037 535 15 0.2% 02, 1 3 0501;, 0. 035 0. 0037 535 15 0.5% 02, 1 8 05012 0. 035 0. 0037 535 15 1.0% Oz, 1 8

Table ICont1nued Weight Sinter- Photoof photoing time Starting Final Example conducconduc- Weight Temperat peak Composition of ambient atparticle sintered N 0. tor ma- Flux tor maof flux ature tempermosphcrc size (miparticle terial torlal (grams) C.) ature crons) size (ml- (grams) (minorons) utes) CdCl, 0. 035 0.0037 535 15 1.7% Oz, 98.3% N1 1 8 CdCl; 0. 035 0.0037 575 10 0.2% Oz, -1 8 CdClz 0.035 0. 0037 575 10 0.5% Oz, 1 8 CdClz 0. 035 0.0037 575 10 1.0% Oz, 1 8

CdCl; 0.035 0.0037 575 10 1.7% Oz, 1 8

CdClB 0.035 0.0037 600 5 0.2% 1 8 OdCl: 0. 035 0. 0037 600 5 0.5% 02,. 1 8

CdClz 0. 035 0.0037 600 5 1.0% Oz, 1 8 CdClz 0. 035 0.0037 600 5 1.7% Oz, 1 8 CdClz 0.035 0. 0084 522 20 0.2% Oz, 1 10 C(lGlz 0. 035 0. 0084 522 20 0.5% 02, 1 10 CdClz 0.035 0. 0084 522 20 1.0% Oz, 1 10 CdClz 0.035 0. 0084 522 20 1.7% Oz, 1 10 CdClz 0.035 0. 0084 535 0.2% 02, 1 10 CdClg 0. 035 0. 0084 535 15 0.5% Oz, 1 10 C(lClz 0.035 0. 0084 535 15 1.0% Oz, 1 10 CdOl 0. 035 0. 0084 535 15 1.7% 02, 1 10 OdOlz 0.035 0. 0084 575 10 0.2% 0 99.8% N2 1 10 CdClz 0. 035 0. 0084 575 10 0.5%-Oz, 1 10 CdCl; 0.035 0. 0084 575 10 1.0% 02, 99.0% N, 1 10 C(lClz 0.035 0. 0084 575 10 1.7% 0 98.3% N2 1 10 each 0.035 0. 0084 600' 5 0.2% 0:, 99.8% N 1 10 CdClz 0.035 0.0081 600 5 0.5% Oz, 90.5% 1 10 CdCh 0. 035 0. 0084 600 5 1.0% 02, 1 10' C(lClz 0. 035 0. 0084 600 5 1.7% Oz, 1 10 EXAMPLE 49 size at all the O -N 211111316111; atmosphere conccntra- (CdSe fiuxed with Cdlsr i 535 C., 10 minutes at 575 C. and 5 minutes at 600 C. A 10% liux content with the same temperatures and sintering times yields an 8 micron particle size-and for a flux content using the same sintering temperaturesand times, a 10 micron particle size. is obtained.

EXAMPLE 50 (CdSe mixed with C111 The processes of Examples 148 are again repeated using Cdl as the flux. The eutectic temperature in this system, CdI CdSe, is 370"" C. The useful sintering range is 370-500 C. for minutes to 2 hours, and a useful flux content range is 5-20 mole percent Cdl For a 5 mole percent flux content at all O -'N ambient atmosphere compositions shown in Examples1-48, a 4 micron size is obtained when sintering is effected for 2 hours at 370 C., 1 hour at 450 C. and 30 minutes at 500 C. For these same sintering times and temperatures a 6 micron particle size is achieved with a flux content of 10 mole percent Cdl and an 8 micron particle size is achieved with a flux content of 20 mole percent Cdl EXAMPLE 51 cds auxed with CdCl The process of Examples 1-48 was repeated except CdS was used in place of CdSe and the conditions were changed as indicated below. The eutectic temperature of the system, CdS-CdCh, is 540 C. The useful sintering range is 540600 C. for 5 to 15 minutes. When .035 gram of CdS and 5, 10, or 20 mole percent of the flux relative to this quantity of CdS (.0024 g., .0049 g. and .0112 g. respectively) are heated at 540 C. and 600 C., the following relationship between particle size, sintering time and sintering temperature was obtained:

For 5 mol percent flux content, 15 minutes at 540 C. or 5 minutes at600 C. yielded a 6 micron particle particle size.

tions(.2% U 99.8% N 5% G 99.5% N 1.0%

O 99.0%N and 1.7% 98.3%' N of Examples 1-48.

At 10 mole percent flux content with the same temperatures and times, etc., an'8 micron particle size was obtained, and at 20 mole percent'flux content, a 10 micron EXAMPLE 52 (CdS fluxed with CdBr The process of Example 49 was repeated except that CdS was used in place of CdSe and the conditions were changed as indicated below. The eutectic temperature in-the system, CdS--CdBr is 548C. The useful sintering range is 548 C.-600 C. for 5 to 15 minutes re spectively. The useful CdBr flux range is 520 mole percent.

5 mole percent CdBr flux at 548 C. for 15 minutes or 600 C. for 5 minutesyields a particle size of 6 microns. For 10 mole percent CdBr flux at the same temperatures'for the same time, an 8 micronsintered particle size-is obtained for and 20 mole percent CdBr flux content, a 10 micron sintered particle size. All of these particle sizes are independent of the O N ambient atmosphere concentrations shown in Example 49.

EXAMPLE 53 (CdS fluxed with Cdl 'The process of Example was repeated except that CdS was used in place of CdSe and the conditions were changed as indicated below. The eutectic temperature in the system, CdS-Cd1 is 380 C. with a useful sintering range of 380500 C. and atime range of 30 minutes-2 hours. For .035 g. of CdS with flux contents of 5, 10 and 20 mole percent, the following relationship between particle size, sintering time and sintering temperature was obtained.

For 5 mole percent flux, 2 hours at 380 C. or 30 minutes at 500 C. yielded a 4 micron sintered particle size at all O -N concentrations'shovvn in Example 50. At 10 mole percent flux, 2 hours at 380 C. and 30 minutes at 500 C. yielded a 6 micron particle size, and at 20 mole percent flux for the same temperatures and times, an 8 micron sintered particle size.

In the previous Examples 1- 53, flux partial pressure was more easily controlled since the photoconductor material to be sintered contained sufficient flux for both the back pressure and the sintering itself. The following examples illustrate the other methods which may be used to control the flux partial pressure and make it also self-adjusting in the situations where there is not sufficient flux in the photoconductor material to be sintered to provide both back pressure and the sintering requirements. Thus, when sample containers of the same volume as those used in Examples 1-53 are employed, but with smaller quantities of potoconductor material and therefore smaller quantities of flux, secondary flux control sources are provided in order to obtain eifective sintering.

EXAMPLE 54 The processes of Examples 1-48 were repeated except that 0.018 g. CdSe were used. This CdSe was mixed with CdCl so that the resulting mixture contained 10 mole percent CdCl by weight. The mixture was affixed to a glass substrate in 7 rectangular shapes of equal surface area. The slide is placed face down on a container having a volume of 1 /2 cubic inches such that the entire area of the substrate upon which the material to be sintered is located faces into the interior of the container. In the bottom of this container, 1 g. of CdCl had been fused. This container was then subjected to ambient atmosphere compositions, and the temperatures for the time intervals given in Examples 1-48. A uniform particle size of 10 microns was obtained as compared with a particle size of 1 micron for the starting material.

The same quantity of CdSe with either or 20 mole percent CdCl subjected to the same conditions yielded a particle size of microns. CdSe fiuxed with CdBr or CdIz in amounts of 5, 10 or 20 mole percent when subjected to the same conditions yielded a particle size of 10 microns also as contrasted with particle size of 1 micron for starting material.

EXAMPLE 55 The processes of Examples 51-53 were repeated except .018 g. of CdS was used and 1 g. of the appropriate flux (CdCl or CdBr or Cdl was fused in the bottom of the container. When the same temperatures, time intervals, flux concentration, ambient atmosphere compositions etc. of Examples 51-53 were applied, the process yielded a sintered photoconductor material having a particle size of 10 microns.

EXAMPLE 56 The processes of Examples 1-50 are repeated except that 0.009 g. of CdSe are used and 1 g. of the appropriate flux is fused in the bottom of the sample container and covered by a A" bed of A1 0 of 200 mesh size (U.S. Standard Sieve). At all the appropriate ambient atmosphere, temperature, time and flux content parameters of these examples a 10 micron particle size was achieved.

EXAMPLE 57 The processes of Examples 51-53 are repeated except that 0.009 g. of CdS are used and 1 g. of the appropriate fiux is fused in the bottom of the sample container and covered by a 4" bed of A1 0 of 200 mesh size (US. Standard Sieve). At all the appropriate ambient atmosphere, temperature, time and flux content parameters of these examples a 10 micron particle size was achieved.

EXAMPLE 58 The processes of Examples 1-50 are repeated except that 0.009 g. of CdSe was used and 1 g. of a mixture having the identical composition as the sintering mixture was fused at the bottom of the sample container and covered by a 4" bed of 200 mesh A1 0 At all the appropriate ambient atmosphere, temperature, time and flux content parameters of these specific examples a 10 micron particle size was achieved.

l. 0 EXAMPLE 59 The processes of Examples 51-53 are repeated except that 0.009 g. of CdS was used and 1 g. of a mixture having the identical composition as the sintering mixture was fused at the bottom of the sample container and covered by a A" bed of 200 mesh A1 0 At all the appropriate ambient atmosphere, temperature, time and fiux content parameters of these specific examples a 10 micron particle size was achieved.

EXAMPLE 60 The processes of Examples 1-50 were repeated except that .035 g. of CdSe are used and 1 g. of the particular flux mixed with the CdSe is fused in the bottom of the sample container. At all the appropriate ambient atmosphere, temperature, time and flux content parameters of these examples a 20 micron particle size was obtained. If the fused flux beds were covered by A" 200 mesh A1 0 a particle size of 15 microns was obtained.

EXAMPLE 61 The processes of Examples 51-53 were repeated except that .035 g. of CdS are used and 1 g. of the particular flux mixed with the CdS is fused in the bottom of the sample container. At all the appropriate ambient atmosphere, temperature, time and fiux content parameters of these examples a 20 micron particle size was obtained. If the fused flux beds were covered by A" 200 mesh A1 0 a particle size of 15 microns was obtained.

EXAMPLE 62 The processes of Examples 54-61 are repeated except that the flux material in the bottom of the sample container is not fused, but is present as 200 mesh (US. Standard Sieve) powders. In those examples where the flux beds are not covered by A1 0 a particle size 10 microns larger than particle size of the specific example was obtained. If the flux beds are covered by a A" 200 mesh A1 0 bed then the particle size is 5 microns larger than the particle size of the specific example.

EXAMPLE 63 The processes of Examples 1-62 are repeated except that solid solutions of CdSe and CdS are used in place of CdSe or CdS. All the other parameters of the process remain the same. The particle size achieved is the same as that obtained for the CdSe or CdS sintered photoconductor layers.

The process of the invention describes methods for controlling the partial pressure of flux used during the sintering cycle of photoconductor materials as well as the effective temperature and time parameters for several fluxes and photoconductor materials. It also sets forth a method of preparing compensated photoconductors using a oxygen-nitrogen ambient sintering atmosphere without the formation of a surface oxide phase that would yield nonohmic devices.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

l. A process for controlling flux concentration in a sintering mixture comprising the material to be sintered plus a volatile flux, volatile at temperatures during which a sintering process is to be effected, which process comprises regulating the fiux vapor pressure in the ambient atmosphere over the sintering mixture during heating at all temperatures involved in the sintering cycle so that the flux concentration in the sintering mixture is maintained at approximately its initial concentration during the entire sintering cycle.

2. A process for controlling flux concentration in a sintering mixture comprising the material to be sintered plus a volatile flux, volatile at temperatures during which a sintering process is to be effected, which process corncentration in the sintering mixture appreciably below its initial concentration during the entire sintering cycle.

3. A process for controlling flux concentration in a sintering mixture comprising the material to be sintered plus a volatile flux, volatile at temperatures during which a sintering process is to be effected, which comprises fulfilling the flux partial pressure requirements in the sample container from a bed of flux material rather than from the sintering mixture so that the flux concentration in the sintering mixture is maintained at approximately its initial concentration during the entire sintering cycle.

4. A process for controlling flux concentration in a sintering mixture comprising the material to be sintered plus a volatile flux, volatile at temperatures during'wh'ich a sintering process is to be effected, which process cornprises fulfilling the flux partial pressure requirements in the sample container, from a bed of flux material overlaid with inert material composed of small particles through which the ilux vapor can diffuse in a regulated manner rather than from the sintering mixture so that flux con centration in the sintering mixture is maintained at approximately its initial'concentration during entire sintering cycle. 7

5. A process for controlling flux concentration in a sintering mixture comprising the material to be sintered plus a volatile flux, volatile at temperatures during which a sintering temperature is to be effected, which process comprises fulfilling fiux partial pressure requirements in the sample container from a separate source of flux in a bed composed of flux plus material to be sintered which has a vapor pressure equal to that of the flux in the sintering mixture rather than from the sintering mixture that the flux concentration in the sintering mixture is maintained at approximately its initial concentration during the entire sintering cycle.

6. A process for controlling flux concentration in a sintering mixture comprising the material to be sintered plus a-volatile flux, volatile at temperatures during which a sintering process is to be effected, which process comprises fulfilling flux partial pressure requirements in sample container from a bed of finely divided flux overlaid with inert material composed of small particles through which flux vapor can diffuse in regulated manner rather than from the sintering mixture so that fiux concentration in the sintering mixture is maintained at approximately its initial concentration during the entire sintering cycle.

7. A process for controlling flux concentration in a sintering mixture comprising the material plus a volatile flux, volatile at temperatures during which a sintering temperature is to be effected, which process'comprises' fulfilling flux partial pressure requirements in sample container from a bed of finely divided flux rather than from the sintering mixture so that flux concentration in the sintering material is maintained at approximately its initial concentration during the entire sinteringcycle.

8. A process for controlling flux concentration in a sintering mixture comprising the material to be sintered plus a volatile flux, volatile at temperatures during which a sintering processis to be effected, which process com prises fulfilling fiux partial pressure requirements in sam- V ple container from a bed of fused flux overlaid with inert material composed of small particles through which the flux vapor can diffuse in regulated manner rather than from the sintering mixture so that the flux concentration in the sintering mixture is maintained at approximately its initial concentration during the entire sintering cycle.

9. The process of claim 1 in which the material to be sintered is a photoconductor material selected from the group consisting of cadmium sulfide, cadmium selenide and 'solid solutions of cadmium sulfide and cadmium selenide.

10. The process of claim 1 in which the flux is selected from the group consisting of cadmium iodide, cadmium bromide and cadmium chloride.

11. A process for controlling the flux partial pressure during the fabrication of fluxed sintered layer photoconductors in a controlled gaseous ambient atmosphere which comprises regulating the flux partial pressure in the ambient atmosphere over a. mixture of the sintering photoconductor material plus flux during the heat treatment at all temperatures involved in the sintering cycle so that the flux concentration of the sintering mixture is maintained at approximately its initial concentration during the entire sintering cycle.

12. A process for fabricating sintered layer photoconductors which comprises:

(1) doping an unsintered photoconductor material by mixing an impurity with said unsintered photoconductor material;

(2) mixing the doped material with a flux material;

(3) arlixing this mixture to a suitable inert substrate;

(4) subjecting the substrate and the unsintered mixture afiixed thereto to a sintering temperature cycle in an oxygen-nitrogen gas atmosphere for a time suflicient to accomplish grain growth to a desired size;

(5) controlling the flux vapor pressure in the ambient atmosphere over the mixture of sintering photoconductor material plus flux during the heat treatment at all temperatures in the sintering cycle so that the flux concentration of the sintering mixture is maintained at approximately its initial concentration during the entire sintering cycle;

(6) thereafter cooling.

13. A process for fabricating sintered layer photoconductors which comprises:

(1) doping an unsintered photoconductor material by mixing an impurity with said unsintered photoconductor material;

(2) mixing the doped unsintered photoconductor material with suilicient flux so that the flux will con- 'stitute 5 to 20 mole percent of the total mixture;

(3) affixing this mixture to an inert substrate;

(4) heating the substrate and the unsintered mixture in a 0.2% 0 99.8% N -1.7% 0 88.3% N gas mixture to a temperature whose minimum value is determined by the eutectic temperature in flux-photoconductor system and whose maximum value is from 80150 C. above the eutectic temperature for time sufficient to accomplish grain growth to a desired particle size;

(5) regulating the flux vapor pressure in the ambient atmosphere above the mixture of sintering photoconductor material plus flux during heat treatment at all temperatures in the sintered cycle so that a flux concentration is maintained at approximately its initial concentration during the entire sintering cycle;

(6) and thereafter cooling.

14. The process of claim 13 in which the regulating of the flux vapor pressure in the ambient atmosphere is 7 accomplished by selecting the volume of the container commensurate to the quantity of flux present in the sintering mixture such that only a small fraction of this flux concentration is required to fulfill the flux partial vapor pressure requirement at the sintering temperature and placing the substrate with the unsintered materials upon a volume restricting container such that the material to be sintered faces into the container.

1'5. The process of claim 13 whereby the flux vapor pressure in the ambient atmosphere is controlled by placing a fused layer of the flux at the bottom of the container to provide an independent source of flux partial vapor pressure.

16. The process of claim 13 in which the flux vapor ering this fused mixture with an inert finely divided ma- 10 terial so as to provide a vapor pressure equal to that of the flux in the sintering mixture.

18. The process of claim 13 in which the photoconductor material is a material selected from the group consisting of cadmium sulfide, cadmium selenide and solid solutions of cadmium sulfide and cadmium selenide.

19. The process of claim 18 in which the flux is selected from the group consisting of cadmium iodide, cadmium bromide and cadmium chloride.

No references cited. 

12. A PROCESS FOR FABRICATING SINTERED LAYER PHOTOCONDUCTORS WHICH COMPRISES: (1) DOPING AN UNSINTERED PHOTOCONDUCTOR MATERIAL BY MIXING AN IMPURITY WITH SAID UNSINTERED PHOTOCONDUCTOR MATERIAL; (2) MIXING THE DOPED MATERIAL WITH A FLUX MATERIAL; (3) AFFIXING THIS MIXTURE TO A SUITABLE INERT SUBSTRATE; (4) SUBJECTING THE SUBSTRATE AND THE UNSINTERED MIXTURE AFFIXED THERETO TO A SINTERING TEMPERATURE CYCLE IN AN OXYGEN-NITROGEN GAS ATMOSPHERE FOR A TIME SUFFICIENT TO ACCOMPLISH GRAIN GROWTH TO A DESIRED SIZE; (5) CONTROLLING THE FLUX VAPOR PRESSURE IN THE AMBIENT ATMOSPHERE OVER THE MIXTURE OF SINTERING PHOTOCONDUCTOR MATERIAL PLUS FLUX DURING THE HEAT TREATMENT AT ALL TEMPERATURES IN THE SINTERING CYCLE SO THAT THE FLUX CONCENTRATION OF THE SINTERING MXITURE IS MAINTAINED AT APPROXIMATELY ITS INITIAL CONCENTRATION DURING THE ENTIRE SINTERING CYCLE; (6) THEREAFTER COOLING. 